After drilling a well, the annular space surrounding the casing is generally cemented to consolidate the well, protect the casing, and to isolate geological layers so as to prevent fluid exchange between the various formation layers. Good hydraulic isolation is thus the primary objective of cementing operations. However, problems can arise during the cementing operations, during cement hydration (including setting) or during the life of the well. For example, problems can arise with the cementing operation such as: (i) design/execution (e.g., mixing); (ii) cement placement in the wellbore (e.g., fluid channels); and (iii) loss of cement to the surrounding formation (e.g., zones without cement). Problems can also arise during cement hydration, such as: (i) liquid to solid transition (e.g., material properties); and (ii) initial stress state. Finally, problems can arise during the life of the well which may include: (i) bulk shrinkage/expansion of the cement sheath; (ii) temperature variations in the casing and/or formation; (iii) pressure variations in the casing and/or formation; and (iv) far-field stress variations (e.g., compaction, activation of shear faults).
Problems such as those outlined above can lead to the formation of fluid channels, radial cracks, disc cracks and micro-annuli in the cement which can in turn compromise well integrity. Consequently, the selection of an optimal cement formulation is important to ensure proper fluid properties for placement of the cement and material properties of the cement after it sets inside the annular region between casing and formation. This is challenging due to the wide range of environmental conditions encountered downhole, such as extremes of temperature and pressure.
Several different methods are currently used to characterize the behavior of cement formulations. These include consistometer tests to measure the thickening time of freshly-mixed slurries at different temperatures, ultrasonic cement analyzer (UCA) tests to get an empirical estimate of the compressive strength development during the first several days of curing at different temperatures and pressures, and split-ring tests to measure bulk volume changes (shrinkage or expansion) during hydration. These tests are well understood and used throughout the oilfield industry for engineering of cement formulations. Each test measures one or two specific aspects of the cement behavior, and the results of several tests are generally combined to give an overall idea of how the cement will perform.